The origin of close massive binaries in the M17 star-forming region

Abstract

Context. Spectroscopic multiplicity surveys of O stars in young clusters and OB associations have revealed that a large portion (∼70%) of these massive stars (Mi > 15 M⊙) belong to close and short-period binaries (with a physical separation of less than a few astronomical units). Follow-up VLT(I) high-angular-resolution observations led to the detection of wider companions (up to d ∼ 500 au), increasing the average companion fraction to > 2. Despite the recent and significant progress, the formation mechanisms leading to such close massive multiple systems remain to be elucidated. As a result, young massive close binaries (or higher-order multiple systems) are unique laboratories for determining the pairing mechanism of high-mass stars. Aims. We present the first VLTI/GRAVITY observations of six young O stars in the M17 star-forming region (≲1 Myr) and two additional foreground stars. VLTI/GRAVITY provides the K-band high-angular-resolution observations needed to explore the close environment of young O-type stars, and, as such, offers an excellent opportunity to characterise the multiplicity properties of the immediate outcome of the massive star formation process. Methods. From the interferometric model fitting of visibility amplitudes and closure phases, we search for companions and measure their positions and flux ratios. Combining the resulting magnitude difference with atmosphere models and evolutionary tracks, we further constrain the masses of the individual components. Results. All six high-mass stars are in multiple systems, leading to a multiplicity fraction of 100% and yielding a 68% confidence interval of 94–100%. We detect a total of nine companions with separations of up to 120 au. Including previously identified spectroscopic companions, the companion fraction of the young O stars in our sample reaches 2.3 ± 0.6. The derived masses span a wide range, from 2.5 to 50 M⊙, with a great tendency towards high-mass companions. However, we do not find a significant correlation between the mass of the companions and their separation. Conclusions. While based on a modest sample, our results clearly indicate that the origin of the high degree of multiplicity is rooted in the star formation mechanism of the sample stars. No clear evidence for one of the competing concepts of massive star formation (core accretion or competitive accretion) could be found. However, given that we find all of the companions within ∼120 au, our results are compatible with migration as a scenario for the formation of close massive binaries.

The onset of stellar multiplicity in massive star formation: A search for low-mass companions of massive young stellar objects with L′-band adaptive optics imaging

“…They report a rather low MF of 17 +21 −17 %. These statistics are in stark contrast with those of Gravity Collaboration et al (2018) and Bordier et al (2022) who report an MF of 100% for relatively small samples of pre-main-sequence massive stars (4 and 6 stars respectively), for similar traced separations. The latter results are consistent with the more evolved Article number, page 9 of 24 A&A proofs: manuscript no.…”

“…We further investigate our findings for a specific class of objects: massive stars. To do so, we relax the evolutionary stage parameter and compare our results to those of different studies (Apai et al 2007;Sana et al 2014;Gravity Collaboration et al 2018;Pomohaci et al 2019;Koumpia et al 2021;Rainot et al 2022;Bordier et al 2022). The results are shown in Fig.…”

“…Similarly, a low-mass object would not significantly affect the orbit of a higher-mass primary, falling off the radial velocities detection limits. In the scenario in which there is a dearth of spectroscopic companions as it was shown in M17 (Sana et al 2017;Ramírez-Tannus et al 2021), the companions lying at intermediate separations (Gravity Collaboration et al 2018;Bordier et al 2022, as observed by) can eventually experience an inward migration on Myr timescales, as theorised by Sana et al (2017). On the other hand, most interferometric studies, whether conducted with ALMA or VLTI, converge towards rather high multiplicities in the 1-200 au scales, (Wheelwright et al 2010;Gravity Collaboration et al 2018;Bordier et al 2022), with the exception of the study conducted by Koumpia et al (2021), for which the MF is lower but the evolutionary stage differs slightly as their objects are all MYSOs.…”

Imaging Molecular Outflow in Massive Star-forming Regions with HNCO Lines

“…G015.03-00.67 is located at the edge of the ionization front of M17 (Chini et al 1980). Blueshifted emissions of HNCO (4-3) and HCO + (1-0) are dominant in the region, which could be influenced by the young stellar objects (YSOs) identified by Spitzer (Povich et al 2009) and Chandra (Broos et al 2007) and/or the OB stars in the open cluster NGC 6618 toward the H II regions (Chini et al 1980;Hanson et al 1997;Hoffmeister et al 2008;Bordier et al 2022). A likely driving source is AGAL015.029-00.669, which is associated with water and Class I methanol masers and revealed to show outflow in CO (Navarete et al 2019).…”

Visual Orbits of Spectroscopic Binaries with the CHARA Array. IV. HD 61859, HD 89822, HD 109510, and HD 191692

“…We continue our paper series of determining the visual orbits of spectroscopic binary stars with long baseline interferometry -Paper I (Lester et al 2019a), Paper II (Lester et al 2019b), and Paper III (Lester et al 2020)-in order to determine the fundamental stellar parameters of the components. Precise fundamental parameters of binary systems are essential for determining orbital demographics (Raghavan et al 2010;Bordier et al 2022), testing models of stellar structure and evolution (e.g., Claret & Torres 2018;Morales et al 2022), calibrating mass and distance determination methods for single stars (Torres et al 2010;Chaplin & Miglio 2013;Gallenne et al 2018), and studying how binary stars form and evolve (e.g., Richardson et al 2021). In this paper, we present the results for the more massive binaries, HD 61859, HD 89822, HD 109510, and HD 191692.…”